This invention relates in general to color television picture tubes, and in particular to an improved process for providing an electrical path between certain components within the picture tube envelope.
A color television cathode ray picture tube commonly comprises a glass bulb including a face panel sealed to the flared end of a funnel. A tubular neck extends from the funnel for receiving an electron gun which provides one or more electron beams. The tube face panel has a concave inner surface upon which is deposited groups of phosphors which are excited to luminescence by an electron beam or beams from the gun. A color selection electrode commonly called a "shadow mask" is attached in precise relationship to the face panel by means of a plurality of suspension members which detachably engage metal studs extending inwardly from a rearwardly extending skirt of the face panel. On the internal surface of the funnel is deposited an electrically conductive coating, usually a composition of a dispersed colloidal graphite termed "dag". A high voltage is applied to this coating from an external power source through an anode button in the funnel wall. This high voltage is commonly termed the ultor anode potential.
An electron-pervious film of aluminum is commonly applied over the electron-excitable phosphor screen; this process is termed "aluminizing". The thickness of the aluminum film is typically about 2,000 angstroms. For maximum brightness of the display, and for brightness uniformity, it is necessary that the aluminum film be as smooth and mirror like as possible, and devoid of blemishes such as holes or blisters. Also, the film must firmly adhere to the phosphor layer and must be uniform in thickness so that the electrons projected by the electron gun can penetrate the coating uniformly. A problem arises in achieving these qualities primarily because of the unsmooth characteristic of the phosphor deposits. The problem is largely resolved by depositing a film of an organic material such as a lacquer on the phosphor deposits before application of the aluminum. The film acts to fill in the rough areas, and provides an uneven surface upon which the aluminum film can be deposited and take on the relatively smooth characteristic of the lacquer.
It is essential for proper cathode ray tube operation that the high voltage on the conductive coating of the funnel be also present on both the aluminum film and on the shadow mask. The electrical path for establishing these components at a common ulter anode potentially commonly comprises a spring extending from the shadow mask which is in contact with a high-voltage-charged funnel coating. The potential on the shadow mask is then conducted to the aluminum film through shadow mask suspension springs which are releasably attached to the metal studs which extend from the face panel flange. The studs, which are embedded in the glass of the flange, are intended to be in electrical contact with the aluminum film; however, this contact often fails to be made.
To insure that the studs are in positive electrical contact with the aluminum film, it has been common practice to paint an electrically conductive bridge "mustache" between the studs and the aluminum coating. The conductive material may comprise a water-soluble silicate in a form suitable for manual application by a brush. The water is driven off in a subsequent tube-baking process, leaving a hard, electrically conductive film between the studs and the aluminum film. Problems are inherent in the use of such films. If, for example, the film does not adhere to the components, fragments of the coating can flake off the studs and the glass area and become migrant particles. Such migrant particles can occlude one or more apertures in the nearby shadow mask; it is to be noted that the occlusion of even one such aperture is highly visible to the viewer. Also, the particles can migrate to the gun area and be the cause of inter-electrode arcing or cathode poisoning. Another problem arises from the manual application of the conductive film, which is commonly painted on by means of a brush. Depending upon the skill of the applicator, the quality can vary from perfect to highly questionable. The problem is aggravated by the fact that manual application is tedious especially under the conditions of a third manufacturing shift.
If the conductive film is of the type that is water-soluble, an extra baking cycle may be necessary to make the film adherent. After aluminizing and before applying the conductive film, it has been necessary to bake the tube in a separate step at a temperature of about 400 degrees Centigrade to eliminate the organic film by oxidizing it. This requirement for a separate bake merely to promote adherence of the conductive film is a costly and energy-wasting step as the film can as well be removed by a subsequent frit cycle bake. Alternatively, in lieu of a separate bake to remove the organic film, the film can be removed by solvents. However, the solvent process is an added expense, and the use of solvents, which are usually inflammable, introduces new problems in the cycle of manufacture.